To encrypt a file we are going to use gpg software. That software is a command line executable software. First you need to install it if it is not installed before in your pc. To do this open a terminal and type:
sudo apt-get install gpg
After that compress the file you want to encyrpt. And type in the terminal
gpg -c fodername.tar.gz
and then type your password twice. It's done :). Delete the file you encrypted. Use foldername.tar.gz.gpg...
